Evolution is often mistakenly depicted as linear in popular culture. One main feature of this depiction in popularculture, but even in science popularisation, is that some ocean-dwelling animal sheds its scales and fins and crawls onto land.

Of course, this showcases only one ancestral lineage for one specific species (Homo sapiens). My question is: Where else did life evolve out of water onto land?

Intuitively, this seems like a huge leap to take (adapting to a fundamentally alien environment) but it still must have happend several times (separately at least for plants, insects and chordates, since their respective most recent common ancestor is sea-dwelling). In fact, the more I think of it the more examples I find.

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I presume you mean how many times did life emerge from the ocean? ("how often" implies you want to know the regularity). Anyway, great question. I really enjoyed reading and thinking about it.

I doubt we know the precise number, or even anywhere near it. But there are several well-supported theorised colonisations which might interest you and help to build up a picture of just how common it was for life to transition to land. We can also use known facts about when different evolutionary lineages diverged, along with knowledge about the earlier colonisations of land, to work some events out for ourselves. I've done it here for broad taxonomic clades at different scales - if interested you could do the same thing again for lower sub-clades.

As you rightly point out, there must have been at least one colonisation event for each lineage present on land which diverged from other land-present lineages before the colonisation of land. Using the evidence and reasoning I give below, at the very least, the following 9 independent colonisations occurred:

bacteria

cyanobacteria

archaea

protists

fungi

algae

plants

nematodes

arthropods

vertebrates

Bacterial and archaean colonisation
The first evidence of life on land seems to originate from 2.6 (Watanabe et al., 2000) to 3.1 (Battistuzzi et al., 2004) billion years ago. Since molecular evidence points to bacteria and archaea diverging between 3.2-3.8 billion years ago (Feng et al.,1997 - a classic paper), and since both bacteria and archaea are found on land (e.g. Taketani & Tsai, 2010), they must have colonised land independently. I would suggest there would have been many different bacterial colonisations, too. One at least is certain - cyanobacteria must have colonised independently from some other forms, since they evolved after the first bacterial colonisation (Tomitani et al., 2006), and are now found on land, e.g. in lichens.

Protistan, fungal, algal, plant and animal colonisation
Protists are a polyphyletic group of simple eukaryotes, and since fungal divergence from them (Wang et al., 1999 - another classic) predates fungal emergence from the ocean (Taylor & Osborn, 1996), they must have emerged separately. Then, since plants and fungi diverged whilst fungi were still in the ocean (Wang et al., 1999), plants must have colonised separately. Actually, it has been explicitly discovered in various ways (e.g. molecular clock methods, Heckman et al., 2001) that plants must have left the ocean separately to fungi, but probably relied upon them to be able to do it (Brundrett, 2002 - see note at bottom about this paper). Next, simple animals... Arthropods colonised the land independently (Pisani et al, 2004), and since nematodes diverged before arthropods (Wang et al., 1999), they too must have independently found land. Then, lumbering along at the end, came the tetrapods (Long & Gordon, 2004).

Note about the Brundrett paper: it has OVER 300 REFERENCES! That guy must have been hoping for some sort of prize.

This is explained at the linked paper, but it's interesting to notice that there are four different arthropod colonizations of land: myriapods, insects, arachnids, and isopods.
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Noah SnyderJun 9 '12 at 17:30

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What is also cool is that some animals (pulmonate molluscs, mammals, etc) have re-colonized the ocean back from the land.
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beroeAug 29 '13 at 23:55

75% of questions about the awesomeness of life, the answer includes tardigrades.
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BobStein-VisiBoneMar 26 '13 at 16:25

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The venomous snakes of Australia are all in a single family, Elapidae. These are thought to have descended from a single sea-snake colonization event, so the sequence would be sea->land->sea->land.
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Chinmay KanchiFeb 9 '14 at 9:07

@ChinmayKanchi Fascinating! If it turns out snakes originally emerged in the sea that'd be sea->land->sea->land->sea->land! (Though my understanding is that currently early snakes being burrowers is more likely.)
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Noah SnyderFeb 9 '14 at 16:33

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It's highly likely that all extant reptiles derive from a terrestrial ancestor because of the egg physiology. As far as I'm aware, not a single reptile lays eggs that can hatch in water.
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Chinmay KanchiFeb 9 '14 at 17:36

@ChinmayKanchi: I'm not sure how that's relevant. Certainly basal amniotes were land-dwellers (that's the first land). But early snakes may have been aquatic (and closely related to Mosasaurs) which would give the second sea. Then basal Elapids would then give the second land, the sea snakes the third sea, and the Australian Elapids the third land. Anyway, as I said current evidence is towards a fossorial origin of snakes against an aquatic origin.
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Noah SnyderFeb 9 '14 at 17:55